Back to EveryPatent.com
United States Patent |
5,714,400
|
Hirao
,   et al.
|
February 3, 1998
|
Method for forming a memory device by utilizing variations in resistance
value
Abstract
On an insulating substrate are formed first aluminum interconnections. In
openings formed in a silicon dioxide film are formed unit cells each
consisting of a tungsten electrode and an aluminum alloy electrode
containing silicon. Over the silicon dioxide film are formed a large
number of linear second aluminum interconnections which are orthogonal to
the first aluminum interconnections. At the individual intersections of
the first and second aluminum interconnections are disposed the unit cells
so as to compose a memory cell array. When a large current is allowed to
flow through the unit cell, silicon in the aluminum alloy electrode moves
in a direction opposite to the current flow and is precipitated in the
aluminum electrode in the vicinity of the interface with the tungsten
electrode, resulting in an increase in resistance value. When a large
current is allowed to flow through the unit cell in the opposite
direction, silicon is diffused, resulting in a reduction in resistance
value. Data can be read by measuring the magnitude of the resistance value
with an extremely small current and judging whether it is in a high state
or in a low state.
Inventors:
|
Hirao; Shuji (Osaka, JP);
Okada; Hideko (Osaka, JP);
Yano; Kousaku (Osaka, JP)
|
Assignee:
|
Matsushita Electric Industrial Co., Ltd. (Osaka, JP)
|
Appl. No.:
|
668179 |
Filed:
|
June 21, 1996 |
Foreign Application Priority Data
Current U.S. Class: |
438/238; 257/E21.606; 257/E27.071; 438/132; 438/468 |
Intern'l Class: |
H01L 021/824.2 |
Field of Search: |
437/918,60,52,919,170
|
References Cited
U.S. Patent Documents
5341328 | Aug., 1994 | Ovshinsky et al. | 365/163.
|
5352621 | Oct., 1994 | Kim et al. | 437/52.
|
Primary Examiner: Tsai; Jey
Attorney, Agent or Firm: McDermott, Will & Emery
Parent Case Text
This is a divisional of application Ser. No. 08/602,285, filed Feb. 16,
1996 U.S. Pat. No. 5,621,247.
Claims
We claim:
1. A method of manufacturing a memory device comprising:
a first step of forming, on a substrate, a plurality of first conductive
members extending in parallel to each other;
a second step of forming, over said first conductive members and said
substrate, an interlayer insulating film;
a third step of forming, in said interlayer insulating film, a plurality of
openings leading to said respective first conductive members;
a fourth step of forming, in each of said respective openings, a resistance
varying member having a resistance varying characteristic such that its
resistance varies depending on a flowing direction and a value of a
current; and
a fifth step of forming, over said interlayer insulating film, a plurality
of second conductive members connected to said respective resistance
varying members and extending in a direction orthogonal to said respective
first conductive members,
wherein said resistance of said resistance varying member is increased when
atoms in said resistance varying member are gathered around an interface
between said first conductive member and said second conductive member
under the influence of a current flowing in a first direction said
resistance of said resistance varying member is decreased when atoms in
said resistance varying member are dispersed within said resistance
varying member under the influence of a current flowing in a second
direction.
2. A method of manufacturing a memory device according to claim 1, wherein
in said fourth step, a first metal electrode and a second metal electrode
are stacked in layers and
either one of said first and second metal electrodes is said resistance
varying member.
3. A method of manufacturing a memory device according to claim 2, wherein
in said fourth step, said first metal electrode is deposited, an insulating
film is deposited over the entire surface of the substrate, said
insulating film is etched back to form sidewalls on side faces of the
openings while said first metal electrode is partially exposed, and said
second metal electrode is formed over that portion of said first electrode
which has been exposed.
4. A method of manufacturing a memory device according to claim 2, wherein
in said second step, a first interlayer insulating film and a second
interlayer insulating film are separately formed,
in said third step, first openings and second openings are separately
formed, and
in said fourth step, the first metal electrodes and the second metal
electrodes are separately formed, wherein
the first interlayer insulating film is formed over said first conductive
members and said substrate, the plurality of first openings leading to
said respective first conductive members are formed in said first
interlayer insulating film, and said first openings are filled with the
first metal electrodes,
the second interlayer insulating film is formed over said first interlayer
insulating film, the second openings are formed in said second interlayer
insulating film, said second openings being shifted in position from said
respective first openings such that said first metal electrodes are
partially exposed, said second openings are filled with the second metal
electrodes to be connected to said first metal electrodes, and
in said fifth step, the plurality of second conductive members are formed
over said interlayer insulating film to be connected to said second metal
electrodes and extend in a direction orthogonal to said first conductive
members.
5. A memory device according to claim 1, wherein
in said first step, the semiconductor substrate is divided into a plurality
of regions by a plurality of isolating insulating films, impurity diffused
layers are formed by introducing an impurity into the regions of said
semiconductor substrate which are located between said adjacent isolating
insulating films, and said impurity diffused layers have their surfaces
silicidized.
6. A method of manufacturing a memory device according to claim 1, further
comprising:
prior to said first step,
the step of forming, on a semiconductor substrate, MOS transistors each
consisting of a gate electrode, a source region, and a drain region;
the step of forming a plurality of bit lines to be connected to the source
regions of said respective MOS transistors; and
the step of depositing, over said MOS transistors and said semiconductor
substrate, an insulating film, wherein
in said first step, the first conductive members are formed on said
insulating film forming said substrate,
said method further comprising
the step of forming contact members for connecting said first conductive
members to the drain regions of said respective MOS transistors.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a memory device utilizing variations in
resistance value which can be used not only as an external memory device
for a computer or the like but also as an internal memory device
incorporated in a ULSI and to a method of manufacturing the same.
With recent remarkable improvements in the performance of a processor or
the like composed of a ULSI to be contained in a computer, an external
memory device having a high capacity has been in increasing demand. To
compose the high-capacity external memory device, such devices as a
magnetic disk, a CD-ROM, and an IC memory device using a silicon
semiconductor have commonly been used.
Among them, a non-volatile memory using fuses as disclosed in U.S. Pat. No.
5,278,784 has particularly been known. FIG. 13 shows a memory cell array
disclosed in the above publication, which consists of a large number of
memory cells arranged to form a matrix. The individual memory cells are
connected by a word line 100 and bit lines 101a to 101c along rows and a
column. In the drawing are shown: select transistors 106a to 106c which
have their gates connected to the above word line 100; auxiliary
transistors 105a and 105b which connect contiguous memory cells; a common
source line 103 connected to each of the sources of the above select
transistors 106a to 106c. Between the memory cells and the bit lines 101a
to 101c are interposed respective fuses 104a to 104c each of which
functions as a non-volatile memory cell. Any one of the fuses 104a to 104c
is melted down by allowing a large current to flow therethrough. If the
disconnected state and connected state of the fuses 104a to 104c are
defined as "1" and "0", respectively, the fuses 104a to 104c can be
utilized as nonvolatile memory (ROM) cells.
However, the conventional magnetic disk, CD-ROM, and non-volatile memory
such as a ROM using fuses have the following drawbacks.
Although the magnetic disk or CD-ROM can be designed to have a larger
capacity, access time is also increased. Consequently, even if the
performance of a computer is to be increased by increasing the capacity of
such a memory, the computer cannot fully achieve its intrinsic
performance. Consequently, an external memory device has presented an
obstacle to improved performance of the computer.
On the other hand, although an IC memory device and the foregoing ROM using
fuses can be accessed at substantially the same speed as a ULSI, chip size
is also increased with increases in capacity, so that manufacturing cost
is increased, resulting in extremely high price.
Moreover, since it is structurally impossible to rewrite to these
non-volatile memories, they only have limited applications. For example, a
non-volatile memory cannot be used as a RAM.
SUMMARY OF THE INVENTION
The present invention has been achieved by focusing attention on a
phenomenon caused by a current allowed to flow between two films composed
of different types of conductive materials, particularly on the phenomenon
that resistance varies when constituent atoms of the conductive materials
move under the influence of the current, which has conventionally been
considered unfavorable. It is therefore an object of the present invention
to provide a novel memory device which utilizes the phenomenon of the
varying resistance in order to perform a storing function and hence has
applications as a variety of memories.
A first memory device of the present invention comprises: a first
conductive member; a second conductive member provided to be opposed to
the above first conductive member; and a resistance varying member
interposed between the above first conductive member and the above second
conductive member, the above resistance varying member having a resistance
varying characteristic such that its resistance varies due to a movement
of atoms which occurs when a current equal to or higher than a specified
value is allowed to flow therethrough and a function of retaining a
high-resistance state and a low-resistance state as data. The above data
can be written in the above resistance varying member by allowing a first
current equal to or higher than the above specified value to flow through
the above resistance varying member and the above data can be read from
the above resistance varying member by allowing a second current lower
than the above specified value to flow through the above resistance
varying member.
The structure enables the writing, reading, and erasing of data utilizing
the resistance varying member having the function of retaining data, while
the data is non-volatile. Moreover, since the resistance varying member is
simple in structure, it can be formed in an extremely small area.
Consequently, if each resistance varying member is used as a unit cell, it
becomes possible to construct a high-density non-volatile memory wherein
data can be written, read, and erased in and from each individual unit
cell. Hence, even when the memory device is increased in integration, its
access time is not increased so that the memory device enables a computer
for which it is used as an external memory to fully achieve its
performance.
In the above first memory device, the above resistance varying member may
be composed of a material having the resistance varying characteristic
such that its resistance varies as a result of the formation of a void in
the vicinity of an end portion thereof due to the movement of the
constituent atoms which is caused by the current equal to or higher than
the above specified value.
Alternatively, the above resistance varying member may be composed of
substantially pure aluminum.
The above first conductive member and the above second conductive member
may be stacked in layers via an interlayer insulating film, the above
interlayer insulating film may be formed with an opening connecting the
above first and second conductive members with each other, and the above
resistance varying member may be an aluminum electrode stacked in layers
in conjunction with a tungsten electrode in the above opening.
The structures enable the writing and reading of data utilizing the
resistance varying characteristic such that the resistance varies due to
the formation of a void in the vicinity of the end portion of the
resistance varying member. Consequently, if each resistance varying member
is used as a unit cell, there can be implemented a non-volatile memory
device wherein data can be written and erased in and from each individual
unit cell.
In the above memory device, a contact area between the above aluminum
electrode and the above tungsten electrode is preferably smaller than a
contact area between the above aluminum electrode and the above first or
second conductive member.
The structure increases the current density at that one of the both ends of
the resistance varying member which has a smaller contact area, so that
the time required to form a void in the end portion can be reduced by
intentionally allowing a current to flow therethrough. As a result, the
time required to write data is reduced, which increases the practicality
of the memory device.
The above data written in the above resistance varying member can be erased
therefrom by switching the high-resistance or low-resistance state of the
above resistance varying member to the opposite state by allowing a third
current equal to or higher than the above specified value to flow in a
direction opposite to the flow of the above first current through the
above resistance varying member via the above first and second conductive
members.
The structure also enables the erasing, i.e., rewriting of data utilizing
the removement of atoms.
The above resistance varying member may be composed of a main material and
an impurity and have the resistance varying characteristic such that its
resistance varies due to the movement of the impurity in the above main
material which is caused by the current equal to or higher than the above
specified value.
Alternatively, the above main material may be aluminum and the above
aluminum may contain 0.5% to 20% of silicon as the above impurity.
The above first conductive member and the above second conductive member
may be stacked in layers via an interlayer insulating film, the above
interlayer insulating film may be formed with an opening connecting the
above first and second conductive members with each other, and the above
resistance varying member may be an aluminum alloy electrode stacked in
layers in conjunction with a tungsten electrode in the above opening.
With the structures, the resistance becomes high when impurity ions are
moved by a current equal to or higher than a specified value and
precipitated at one end of the resistance varying member, while it becomes
low when the impurity ions precipitated at the end of the resistance
varying member are diffused by a current flowing in the opposite
direction. Consequently, if each resistance varying member is used as a
unit cell utilizing variations in resistance value due to the movement of
the impurity atoms, there can be implemented a non-volatile memory device
wherein data can be written, read, and erased in and from each individual
cell, i.e., rewritten therein.
In this case, a contact area between the above aluminum alloy electrode and
the above tungsten electrode may be smaller than a contact area between
the above aluminum alloy electrode and the above first or second
conductive member.
The structure increases the current density at that one of the both ends of
the resistance varying member which has a smaller contact area, so that
the time required to form a void at the end portion can be reduced by
allowing a current to flow therethrough. As a result, the time required to
write data and the time required to erase data is reduced, which increases
the practicality of the memory device.
A second memory device of the present invention is obtained in such a
manner that, in the above first memory device, the above first conductive
member is composed of a plurality of first interconnections extending in
parallel to each other, the above second conductive member is composed of
a plurality of second interconnections extending in a direction orthogonal
to the above respective first interconnections, and the above resistance
varying member is disposed at each of the intersections of the above first
interconnections and the above second interconnections.
The structure enables the resistance varying members of the memory device
to be accommodated at high density in an extremely small area, so that the
cost and size of the memory device can be reduced.
The second memory device may further comprise: a plurality of MOS
transistors each consisting of a semiconductor substrate, a gate
electrode, a source region, and a drain region; and a plurality of bit
lines connected to the source regions of the above respective MOS
transistors, wherein the above first conductive members are impurity
diffusion layers formed by introducing an impurity into a plurality of
linear regions of the above semiconductor substrate, which has been
partitioned by a plurality of isolating insulating films, and having their
surfaces silicidized and the above first conductive members are connected
to the drain regions of the above respective MOS transistors so that data
can be written and read in and from that one of the resistance varying
members selected via the above gate electrode and the above bit line of
each of the above MOS transistors.
In the above second memory device, the above data written in the above
resistance varying member can be erased therefrom by switching the
high-resistance or low-resistance state of the above resistance varying
member to the opposite state by allowing a third current equal to or
higher than the above specified value to flow in a direction opposite to
the flow of the above first current through the above resistance varying
member via the above first and second conductive members and the data can
be erased from the resistance varying member which has been selected via
the above gate electrode and the above bit line of each of the above MOS
transistors and in which the data has been written.
The structure enables a memory unit composed of the resistance varying
members as the unit cells and a peripheral circuit unit containing the MOS
transistors as the select transistors to be mounted on the same
semiconductor substrate. In this case, since the structure of the memory
device can be implemented by using a typical salicide process for MOS
transistors, there can be obtained a memory device which is high in
overall integration and functions as a PROM or RAM manufactured at
extremely low cost.
The second memory device may further comprise: a plurality of MOS
transistors each consisting of a semiconductor substrate, a gate
electrode, a source region, and a drain region; and a plurality of bit
lines connected to the source regions of the above respective MOS
transistors. The above first conductive members are formed over the above
respective MOS transistors via insulating films and the above resistance
varying members are connected to the drain regions of the above respective
MOS transistors via contacts formed in the above insulating films so that
data can be written and read in and from that one of the resistance
varying members selected via the above gate electrode and the above bit
line of each of the above MOS transistors.
The above data written in the above resistance varying member can be erased
therefrom by switching the high-resistance or low-resistance state of the
above resistance varying member to the opposite state by allowing a third
current equal to or higher than the above specified value to flow in a
direction opposite to the flow of the above first current through the
above resistance varying member via the above first and second conductive
members and the data can be erased from the resistance varying member
which has been selected via the above gate electrode and the above bit
line of each of the above MOS transistors and in which the data has been
written.
The structure enables the memory unit composed of the resistance varying
members as the unit cells and the peripheral circuit unit containing the
MOS transistors as the select transistors to be three-dimensionally
accommodated in the same portion of the semiconductor substrate, so that a
memory device which is extremely high in integration and functions as a
PROM or RAM can be obtained.
A method of manufacturing a memory device according to the present
invention comprises: a first step of forming, on a substrate, a plurality
of first conductive members extending in parallel to each other; a second
step of forming, over the above first conductive members and the above
substrate, an interlayer insulating film; a third step of forming, in the
above interlayer insulating film, a plurality of openings leading to the
above respective first conductive members; a fourth step of forming, in
each of the above respective openings, a resistance varying member having
a resistance varying characteristic such that its resistance varies due to
the movement of atoms which occurs when a current equal to or higher than
a specified value is allowed to flow therethrough; and a fifth step of
forming, over the above interlayer insulating film, a plurality of second
conductive members connected to the above respective resistance varying
members and extending in a direction orthogonal to the above respective
first conductive members.
By the method, there can easily be formed a high-integration memory device
wherein each resistance varying member is used as a unit cell and data can
be read and written from and in each individual unit cell.
In the above method of manufacturing a memory device, in the above fourth
step, a first metal electrode and a second metal electrode may be stacked
in layers and either one of the above first and second metal electrodes
may be the above resistance varying member.
By the method, one of the first and second metal electrodes serves as the
resistance varying member, while the other does not serve as the
resistance varying member. If the material and thickness of the metal
electrode not serving as the resistance varying member are properly
selected and determined, a void or atoms can easily and positively be
formed or precipitated, respectively, in the metal electrode serving as
the resistance varying member in the vicinity of the interface with the
metal electrode not serving as the resistance varying electrode when the
atoms are moved by a current, while the capacitance between the conductive
members can be reduced, resulting in the formation of a memory device with
high performance.
In the above memory device, in the above fourth step, the above first metal
electrode may be deposited, an insulating film may be deposited over the
entire surface of the substrate, the above insulating film may be etched
back to form sidewalls on side faces of the openings while the above first
metal electrode is partially exposed, and the above second metal electrode
may be formed over that portion of the above first electrode which has
been exposed.
In the above method of manufacturing a memory device, in the above second
step, a first interlayer insulating film and a second interlayer
insulating film may be separately formed, in the above third step, first
openings and second openings may be separately formed, and in the above
fourth step, the first metal electrodes and the second metal electrodes
may be separately formed, wherein the first interlayer insulating film may
be formed over the above first conductive members and the above
substrate,.the plurality of first openings leading to the above respective
first conductive members may be formed in the above first interlayer
insulating film, and the above first openings may be filled with the first
metal electrodes, the second interlayer insulating film may be formed over
the above first interlayer insulating film, the second openings may be
formed in the above second interlayer insulating film, the above second
openings being shifted in position from the above respective first
openings such that the above first metal electrodes are partially exposed,
the above second openings may be filled with the second metal electrodes
to be connected to the above first metal electrodes, and in the above
fifth step, the plurality of second conductive members may be formed over
the above interlayer insulating film to be connected to the above second
metal electrodes and extend in a direction orthogonal to the above first
conductive members.
By the method, the contact area between the first metal electrode and the
second metal electrode is reduced, so that there can be formed a memory
device which is practically advantageous in that the resistance of the
resistance varying member can be varied promptly by forming a void or
precipitating atoms in the interface having a small contact area when the
atoms are moved.
In the above method of manufacturing a memory device, in the above first
step, the semiconductor substrate may be divided into a plurality of
regions by a plurality of isolating insulating films, impurity diffused
layers may be formed by introducing an impurity into the regions of the
above semiconductor substrate which are located between the above adjacent
isolating insulating films, and the above impurity diffused layers may
have their surfaces silicidized.
The method enables the memory unit composed of the resistance varying
members as the unit cells and the peripheral circuit unit containing the
MOS transistors as the select transistors to be formed on the same
semiconductor substrate through a typical salicide process. As a result,
the memory device can be manufactured at lower cost.
The above method of manufacturing a memory device further comprises: prior
to the above first step, the step of forming, on a semiconductor
substrate, MOS transistors each consisting of a gate electrode, a source
region, and a drain region; the step of forming a plurality of bit lines
to be connected to the source regions of the above respective MOS
transistors; and the step of depositing, over the above MOS transistors
and the above semiconductor substrate, an insulating film, wherein in the
above first step, the first conductive members are formed on the above
insulating film forming the above substrate, the above method further
comprising the step of forming contact members for connecting the above
first conductive members to the drain regions of the above respective MOS
transistors.
The method enables the peripheral circuit unit containing the MOS
transistors as the select transistors and the memory unit composed of the
resistance varying members as the unit cells to be formed
three-dimensionally on the semiconductor substrate. Consequently, there
can be implemented a memory device which is extremely high in integration.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view schematically showing the structure of a
memory device according to a first embodiment;
FIG. 2(a) is a plan view of the memory device according to the first
embodiment and FIG. 2(b) is a cross-sectional view thereof;
FIG. 3 is a cross-sectional view illustrating the principle of writing data
in a unit cell portion of the memory device according to the first
embodiment;
FIGS. 4(a) to 4(d) are cross-sectional views illustrating the process of
manufacturing a memory device according to a second embodiment;
FIGS. 5(a) to 5(d) are cross-sectional views illustrating the process of
manufacturing a memory device according to a third embodiment;
FIG. 6 is a plan view illustrating displacements of individual openings in
the third embodiment;
FIG. 7 is a cross-sectional view of a memory device according to a fourth
embodiment;
FIGS. 8(a) and 8(b) are cross-sectional views illustrating the principle of
writing and erasing data in and from the unit cell portion of the memory
device according to the fourth embodiment;
FIG. 9 is a cross-sectional view of a semiconductor integrated circuit
according to a fifth embodiment;
FIGS. 10(a) and 10(b) are plan views schematically showing the respective
structures of the semiconductor integrated circuit according to the fifth
embodiment and of a typical DRAM;
FIGS. 11(a) to 11(d) are cross-sectional views illustrating the process of
manufacturing a semiconductor memory device according to a sixth
embodiment;
FIG. 12 is a circuit diagram of the semiconductor memory device according
to the sixth embodiment; and
FIG. 13 is a circuit diagram of a conventional non-volatile memory using
fuses.
DETAILED DESCRIPTION OF THE INVENTION
(First Embodiment)
Initially, a first embodiment will be described with reference to FIG. 1,
FIGS. 2(a) and 2(b), and FIG. 3.
FIG. 1 is a perspective view of a memory device according to the first
embodiment. FIG. 2(a) is a plan view of the memory device and FIG. 2(b) is
a cross-sectional view thereof.
On an insulating substrate 1 made of alumina or glass has been provided
first aluminum interconnections 2 each serving as a first conductive
member. The first aluminum interconnections 2 are composed of a large
number of fine lines each having a thickness of about 500 nm and a width
of 500 nm. The fine lines composing the first aluminum interconnections 2
are arranged in parallel with a pitch of 1 .mu.m or less. On the first
aluminum interconnections 2 on the insulating substrate 1 has been
deposited a silicon dioxide film 3 serving as an interlayer insulating
film and having a thickness of about 1000 nm. On the silicon dioxide film
3 have been formed second aluminum interconnections 6 each serving as a
second conductive member. The second aluminum interconnections 6 are
composed of a large number of fine lines each having substantially the
same thickness and width as the above first aluminum interconnection 2.
The fine lines composing the second aluminum interconnections 6 have
substantially the same pitch as the first aluminum interconnections 2 and
extend in the direction orthogonal to the first aluminum interconnections
2. At each of the intersections of the first aluminum interconnections 2
and the second aluminum interconnections 6 on the plane shown in FIG. 2(a)
is disposed a unit cell Mc serving as a varying resistance member. The
unit cell Mc is constituted by a multilayer film consisting of a tungsten
electrode 4 and an overlying aluminum electrode 5, which have sequentially
been deposited in an opening 8 formed in the silicon dioxide film 3. Each
of the tungsten electrode 4 and overlying aluminum electrode 5 has a
thickness of about 400 nm. The above aluminum electrode 5 has a
composition substantially similar to that of pure aluminum. Each of the
first and second aluminum interconnections 2 and 6 contains about 0.5% of
silicon.
Although each of the first and second aluminum interconnections 2 and 6 is
simply represented by lines in FIG. 1 for the sake of clarity, it actually
has a given width and a given thickness, as shown in FIGS. 2(a) and 2(b).
The memory device according to the present embodiment can be fabricated by
utilizing such existing technology as described below, though the drawing
of the fabrication process is omitted here. Initially, an aluminum film
having a thickness of 500 nm is formed by sputtering on the insulating
substrate 1, followed by the formation of the first aluminum
interconnections 2 by photolithography and dry etching. Subsequently, the
silicon dioxide film 3 having a thickness of about 1000 nm is deposited
by, e.g., plasma CVD over the entire surface of the substrate, followed by
planarization. On the planarized silicon dioxide film 3 is formed a
photoresist mask having numerous openings corresponding to the individual
intersections of the first aluminum interconnections 2 and the second
aluminum interconnections 6. Dry etching is then performed using the
photoresist mask so that the openings 8 leading to the first aluminum
interconnections 2 are formed in the silicon dioxide film 3. After the
photoresist mask is removed, tungsten and aluminum are sequentially
deposited on the first aluminum interconnection 2 in each of the openings
8 by selective CVD, thus forming the tungsten electrode 4 and the aluminum
electrode 5 each having a thickness of about 400 nm. Thereafter, an
aluminum film having a thickness of about 500 nm is formed by sputtering
over the entire surface of the substrate, followed by the formation of the
second aluminum interconnections 6 by photolithography and dry etching.
FIG. 3 is an enlarged cross-sectional view showing only one unit cell Mc.
There has been known the phenomenon that, when a current is allowed to
flow in the direction indicated by the arrow in the drawing by application
of a low voltage and a high voltage to the first aluminum interconnection
2 and to the second aluminum interconnection 6, respectively, aluminum
atoms move in the direction opposite to the current flow. The phenomenon
is termed electromigration and generally considered unfavorable since it
lowers the reliability of a semiconductor device.
Here, the present embodiment focuses attention on the fact that the
resistance value is increased by the movement of the aluminum atoms. If a
current on the order of 1.times.10.sup.6 A/cm.sup.2 is allowed to flow
through the unit cell Mc, the aluminum atoms move in the direction
opposite to the current flow, so that voids 7 are formed in the aluminum
electrode 5 in the vicinity of the interface with the tungsten electrode,
which drastically increases the resistance of the unit cell Mc. If the
state in which the resistance value has been increased by the formation of
the voids 7 is defined as data "1" (or "0"), while the state in which the
resistance value remains low with no void 7 formed is defined as "0" (or
"1"), the unit cell Mc can be utilized as a memory cell. By using a
Sufficiently small current equal to or less than 0.5.times.10.sup.5
A/cm.sup.2 in reading the data therefrom, it can be judged whether the
resistance of each unit cell Mc is in the high state or in the low state
without adversely affecting data retention.
In the reading operation, the resistance of the unit cell Mc may vary due
to a data read current. However, if sufficient difference (ratio) is
provided between a data write current and the data read current (e.g., 100
to 1000 times or more), variations in resistance value as a result of the
reading operation can be avoided. In the case where the resistance value
may vary during the reading operation for such a reason that the
difference between the data write current and the data read current cannot
be increased sufficiently, problems could be avoided by adding to the
memory device such a refreshing function as performed in a DRAM. Moreover,
even in the case where refreshing should be performed, stored data is
retained if a power source is turned off, since it is non-volatile.
Consequently, it is sufficient to perform refreshing when memory operation
is unaffected (e.g., when the memory is not used or when another block in
the memory is used). It is not necessary to perform refreshing
simultaneously with reading, as is performed in a DRAM, so that the
reading operation is not adversely affected.
The present embodiment has focused attention on the phenomenon of
electromigration which is observed when a current is allowed to flow
through the aluminum electrode 5 and has utilized such a characteristic
that resistance increases due to voids formed in the aluminum electrode 5
as a result of the electromigration. Since the resulting high state of
resistance is essentially non-volatile, as described above, it is retained
even when the power source is turned off. Moreover, since the formation of
the voids 7 can be considered to be an irreversible process and the
writing of data is possible at any time, the memory device in the present
embodiment functions as a PROM.
The speed at which the voids 7 are formed is dependent on temperature. The
voids 7 are formed more promptly at a higher temperature. Accordingly, by
heating the memory device to about 200.degree. C. prior to the writing
operation, writing time can be reduced.
(Second Embodiment)
Below, a second embodiment will be described with reference to FIGS. 4(a)
to 4(d). FIGS. 4(a) to 4(d) are cross-sectional views illustrating the
process of manufacturing a semiconductor memory device according to the
present embodiment.
Initially, as shown in FIG. 4(a), the first aluminum interconnections 2
composed of a large number of fine lines are formed on the insulating
substrate 1. The size and formation method of the aluminum interconnection
2 are the same as described in the above first embodiment.
Next, as shown in FIG. 4(b), the silicon dioxide film 3 is formed over the
entire surface of the substrate by the same method as used in the above
first embodiment. Subsequently, the openings 8 leading to the first
aluminum interconnections 2 are formed in the silicon dioxide film 3 by
dry etching using a photoresist mask.
Next, as shown in FIG. 4(c), tungsten is deposited on the first aluminum
interconnections 2 in the openings 8 by selective CVD, so as to form the
tungsten electrodes 4 each having a thickness of about 200 nm. Thereafter,
a silicon dioxide film is deposited over the entire surface of the
substrate to a thickness corresponding to about 1/2 of the shortest side
of the opening 8, followed by the formation of insulating sidewalls 10 on
the side faces of the openings 8. At that stage, the tungsten electrodes 4
have partially been exposed.
Next, as shown in FIG. 4(d), aluminum is deposited on these portions of the
tungsten electrodes 4 which have been exposed by selective CVD, so as to
form the aluminum electrodes 5 each having a thickness of 200 nm.
Furthermore, the second aluminum interconnections 6 composed of a large
number of fine lines to be connected to the respective aluminum electrodes
5 are formed, similarly to the above first embodiment. Finally, the above
tungsten electrode 4 and the aluminum electrode 5 constitute the unit cell
Mc, similarly to the above first embodiment.
In the present embodiment also, the resistance value can be increased by
allowing the data write current to flow, similarly to the above first
embodiment, so that data can be stored by utilizing the phenomenon.
Moreover, since the contact area between the tungsten electrode 4 and the
aluminum electrode 5 is smaller than the contact area between the aluminum
electrode 5 and the second aluminum interconnection 6 in the present
embodiment, an effective current density at the contact portion of the
tungsten electrode 4 and the aluminum electrode 5 is increased when the
write current has flown. Consequently, the time required to form the voids
in the aluminum electrode 5 can be reduced.
(Third Embodiment)
Below, a third embodiment will be described with reference to FIGS. 5(a) to
5(d) and to FIG. 6. FIGS. 5(a) to 5(d) are cross-sectional views
illustrating the process of manufacturing a semiconductor memory device
according to the present embodiment.
Initially, as shown in FIG. 5(a), the first aluminum interconnections 2
composed of a large number of fine lines are formed on the insulating
substrate 1. The size and formation method of the aluminum interconnection
2 are the same as described in the above first embodiment. After a first
silicon dioxide film 3a is further deposited to a thickness of about 20 nm
over the entire surface of the substrate, first openings 8a leading to the
first aluminum interconnections 2 are formed in the first dioxide film 3a
by dry etching using the photoresist mask.
Next, as shown in FIG. 5(b), tungsten is deposited on the aluminum
interconnections 2 in the first openings 8a by selective CVD, so as to
form the tungsten electrodes 4 composed of tungsten filled in the first
openings 8a.
Next, as shown in FIG. 5(c), a second silicon dioxide film 3b is deposited
to a thickness of about 200 nm over the entire surface of the substrate,
followed by the formation of second openings 8b in the same manner as used
to form the above first openings 8a. During the process, the present
embodiment has shifted the second openings 8b in position from the first
openings 8a, as shown in FIG. 6.
Next, as shown in FIG. 5(d), the aluminum electrodes 5 are formed in the
second opening 8b, followed by the formation of the second aluminum
interconnections 6 composed of a large number of fine lines to be
connected to the respective aluminum electrodes 5 in the same manner as
used in the above first embodiment. Finally, the above tungsten electrode
4 and the aluminum electrode 5 constitute the unit cell Mc, similarly to
the above first embodiment.
In the present embodiment also, the resistance value can be increased by
allowing the write current to flow, similarly to the above first
embodiment, so that data can be stored by utilizing the phenomenon.
Moreover, since the present embodiment has formed the tungsten electrode 4
and the aluminum electrode 5 such that they are shifted in position from
each other and overlapping only partially, as shown in FIG. 6, the contact
area between the aluminum electrode 5 and the tungsten electrode 4 is
smaller than the contact area between the aluminum electrode 5 and the
second aluminum interconnection 6. As a result, an effective current
density at the contact portion of the tungsten electrode 4 and the
aluminum electrode 5 is increased when the write current has flown,
similarly to the above second embodiment. Consequently, the time required
to form the voids in the aluminum electrode 5 can be reduced so that the
same effect as achieved in the above second embodiment can be achieved.
(Fourth Embodiment)
Below, a description will be given to a fourth embodiment with respect to
FIG. 7, FIGS. 8(a) and 8(b), and FIG. 9.
FIG. 7 is a cross-sectional view showing the structure of a memory device
according to the present embodiment. The structure of the memory device of
the present embodiment is basically the same as that of the memory device
of the above first embodiment, so that the description of their common
components is omitted here. In the present embodiment, there are provided
aluminum alloy electrodes 20 containing about 0.5% to 20% of silicon,
instead of the aluminum electrodes 5 according to the above first
embodiment. Therefore, the tungsten electrode 4 and the aluminum alloy
electrode 20 constitute the unit cell Mc having the function of storing
data in the present embodiment.
In the formation of the aluminum alloy electrodes 20, if they contain about
0.5% of silicon, for example, the composition thereof can be the same as
that of the overlying second aluminum interconnections 6, so that the
aluminum alloy electrodes 20 and the second aluminum interconnections 6
can be formed simultaneously from an aluminum film, which has been
deposited over the openings and the entire surface of the substrate by
sputtering or like technique after the formation of the tungsten electrode
4, and patterned.
To form the aluminum alloy electrodes 20 which contain a large amount of
silicon and hence is different in composition from the second aluminum
interconnections 6, the following methods can be used. For example, pure
aluminum may be deposited on the tungsten electrodes 4 by selective CVD so
that silicon ions are implanted into the pure aluminum. Alternatively, an
aluminum alloy film containing a large amount of silicon may be deposited
not only over the tungsten electrodes 4 but also over the entire surface
of the substrate by sputtering or like technique so that the aluminum
alloy film is etched back by CMP or like technique till the silicon
dioxide film becomes exposed and remains only in the openings. It will be
appreciated that other well-known techniques used to form filled plugs can
also be used in the present embodiment.
FIGS. 8(a) and 8(b) are enlarged cross-sectional views each showing the
unit cell Mc portion. As shown in FIG. 8(a), if a current is allowed to
flow in the direction indicated by the arrow in the drawing by application
of a high voltage and a low voltage to the first aluminum interconnection
2 and to the second aluminum interconnection 6, respectively, silicon 21
in the aluminum alloy electrode 20 moves in the direction opposite to the
current flow, so that the silicon 21 is precipitated in the aluminum alloy
electrode 20 in the vicinity of the interface with the tungsten electrode
4. The precipitation of the silicon 21 drastically increases the
resistance of the unit cell Mc, which indicates that data has been
written, similarly to the above first embodiment.
On the other hand, as shown in FIG. 8(b), if a current on the order of
1.times.10.sup.6 A/cm.sup.2, e.g., is allowed to flow in the direction
indicated by the arrow in the drawing by application of a low voltage and
a high voltage to the first aluminum interconnection 2 and to the second
aluminum interconnection 6, respectively, in the memory device of FIG.
8(a), the silicon 21 in the aluminum alloy electrode 20 moves in the
direction opposite to the current flow, so that the silicon 21 that has
been precipitated in the aluminum alloy electrode 20 in the vicinity of
the interface with the tungsten electrode 4 is diffused into the aluminum
alloy 5, which lowers the resistance of the unit cell Mc and hence enables
the erasing of data.
The reading of data is performed using a sufficiently small current of
0.5.times.10.sup.5 A/cm.sup.2 or less so that it can be judged whether the
data is "0" or "1" based on the magnitude of the resistance value of each
unit cell Mc.
In the reading operation of the present embodiment also, variations in
resistance value as a result of the reading operation can be avoided by
providing a sufficient difference (ratio) (e.g., 100 to 1000 times or
more) between the data write current and the data read current. In the
case where the resistance value may vary during the reading operation for
such a reason that the difference between the data write current and the
data read current cannot be increased sufficiently, problems can be
avoided by adding to the memory device such a refreshing function as
performed in a DRAM. Moreover, even in the case where refreshing should be
performed, stored data is retained if a power source is turned off, since
it is non-volatile. Consequently, it is sufficient to perform refreshing
when memory operation is unaffected (e.g., when the memory is not used or
when another block in the memory is used). It is not necessary to perform
refreshing simultaneously with reading, as is performed in a DRAM, so that
the reading operation is not adversely affected.
The present embodiment has focused attention on the phenomenon that the
silicon 21, which has been moved by the current flowing through the
aluminum alloy electrode containing silicon in the direction opposite to
the current flow, is precipitated in the aluminum alloy electrode 20 in
the vicinity of the interface with the tungsten electrode 4 and has
utilized, as a memory, such a characteristic that the resistance is
increased by the precipitation of the silicon 21 in the aluminum alloy
electrode 20 in the vicinity of the interface with the tungsten electrode
4. Since the high-state and low-state of the resistance are essentially
non-volatile, they are retained even when the power source is turned off.
Moreover, since the movement of the silicon is an irreversible process, as
described above, data can be written in and erased from, i.e., rewritten
in each individual unit cell Mc at any time in the memory device of the
present embodiment. Hence, the memory device in the present embodiment
functions as a non-volatile RAM.
Variations in resistance value caused by the current flowing through each
of the unit cells are different depending on the diameter of the opening
or on the materials of individual components. In the case where the
aluminum electrode is used and the diameter of the opening is 0.6 .mu.m,
for example, the resistance value is varied from 0.2 .OMEGA. to 1.1
.OMEGA. by application of a current. In the case where the diameter of the
opening is 0.2 .mu.m, the resistance value is varied from 2 .OMEGA. to
about 10 .OMEGA. by application of a current.
The speed at which the silicon 21 moves is dependent on temperature. The
silicon is precipitated and diffused at a higher speed at a higher
temperature. Therefore, the time required by writing and erasing
operations can be reduced by heating the memory device to about
200.degree. C. prior to the writing and erasing operations.
The memory device in the present embodiment is similar to a RAM in that
data can be rewritten in each individual unit cell and that random access
can be performed thereto. However, the memory device in the present
embodiment has a functional advantage over the SRAM in that stored data is
retained even when the power source is turned off, while it occupies an
area much smaller than the area occupied by a DRAM which is further
smaller than the area occupied by the SRAM. To be more specific, the unit
cells functioning as memories have a width of about 0.5 .mu.m and a pith
of 1 .mu.m in the present embodiment. If 0.35-.mu.m design rules are
adapted, however, the pitch of the unit cells can easily be reduced to
about 0.7 .mu.m. Compared with the DRAM in which the pitch is 1.7 to 1.9
.mu.m in accordance with the 0.35-.mu.m design rules, the memory device in
the present embodiment occupies only about 30% of the area occupied by The
DRAM. Hence, it can be concluded that the memory device according to the
present embodiment is remarkably advantageous over the SRAM in terms of
the function of retaining stored data and over a DRAM in terms of
achieving higher integration.
In addition, since the memory device in the present embodiment is
considerably simple in structure, it can be miniaturized in conjunction
with prospective miniaturization of a semiconductor integrated circuit.
(Fifth Embodiment)
Below, a fifth embodiment will be described with reference to FIG. 9 and
FIGS. 10(a) and 10(b). FIG. 9 is a cross-sectional view showing the
structure of a semiconductor memory device in the present embodiment and
FIG. 10(a) is a plan view showing the structure thereof. As shown in FIG.
9, the semiconductor memory device consists of a cell select/drive circuit
region 50 and a memory cell region 60.
In the cell select/drive circuit region 50, active regions surrounded by an
isolation 52 are formed on a semiconductor substrate 51. On the
semiconductor substrate 51 in the active regions, respective gate
electrodes 53 are formed via gate insulating films. Into these portions of
the semiconductor substrate 51 located on both sides of the gate
electrodes 53 has been diffused an impurity for forming source/drain
regions 54, so that MOS transistors are formed. The transistor in the
present embodiment has a so-called LDD structure in which sidewalls are
provided on the side faces of the gate electrode and the source/drain
regions 54 consist of low-concentration source/drain regions and
high-concentration source/drain regions. Over the substrate has been
deposited an insulating layer 55 composed of a silicon dioxide film.
The memory cell region 60 in the present embodiment is formed on the
insulating layer 55 and has the same structure as the memory device in the
second embodiment described above. Specifically, the memory cell region 60
comprises: first aluminum interconnections 62 formed on the insulating
layer 55; a silicon dioxide film 63 deposited over the entire surface of
the substrate; tungsten electrodes 64, aluminum electrodes 65, and
sidewalls 67 formed in the respective openings 8 formed in the silicon
dioxide film 63; and second aluminum interconnections 66 formed over the
silicon dioxide film 63 and aluminum electrodes 65.
The MOS transistors in the above cell select/drive circuit region 50 can be
divided into two types depending on their functions: some of them serve as
select transistors, while the others serve as drive transistors. The drain
of the select transistor is in contact with a first plug 56 filled in a
contact hole formed in the insulating layer 55, while the upper end of the
plug 56 is in contact with the lower end of the first aluminum
interconnection 52 in the memory cell region 60. The source of the select
transistor is in contact with a bit line 57 indicated by the broken line
in the drawing at a portion not shown in the cross section of FIG. 9. A
word line extends in the direction orthogonal to the cross section of FIG.
9 over the isolation 52 and active region, while functioning as the gate
electrode 53 in the individual active region. On the other hand, the
source of the leftmost drive transistor in FIG. 9 is in contact with the
second aluminum interconnection 66 of the memory cell region 60, while the
drain thereof is in contact with a power-source terminal via the
corresponding second plug 58.
FIG. 10(a) is a plan view diagrammatically showing the semiconductor memory
device according to the present embodiment. FIG. 10(b) is a plan view
diagrammatically showing the structure of a typical DRAM, which is
provided for comparison with FIG. 10(a). Through the comparison between
FIGS. 10(a) end 10(b), it will easily be understood that, since the cell
select/drive circuit and memory cells can easily be formed in three
dimensions in the semiconductor memory device of the present embodiment,
the area occupied thereby can drastically be reduced compared with the
area occupied by the typical DRAM in which a peripheral circuit is formed
on both sides of memory cells.
(Sixth Embodiment)
Below, a sixth embodiment will be described with reference to FIGS. 11(a)
to 11(d), which are cross-sectional views illustrating the process of
manufacturing a semiconductor memory device according to the sixth
embodiment.
Initially, as shown in FIG. 11(a), isolating oxide films 77 composed of a
large number of linear silicon dioxide films extending in parallel to each
other are formed on a silicon substrate 71. Into these portions of the
silicon substrate 71 corresponding to the spaces between the adjacent
isolating oxide films 77 are implanted phosphorus ions, thereby forming
n.sup.+ diffused layers 72 in stripes. After a titanium film is further
formed over the n.sup.+ diffused layers 72, the titanium is caused to
react with the silicon in the n.sup.+ diffused layers 72, thereby
silicidizing the surfaces of the n.sup.+ diffused layers 72. The
silicidization can be performed by a well-known technique. The n.sup.+
diffused layers 72 have a width of about 0.4 .mu.m and a pitch of about
0.8 .mu.m. The n.sup.+ diffused layer 72 has the same function as the
first aluminum interconnection in the above first embodiment and the like
and forms a first conductive member.
Next, as shown in FIG. 11(b), a silicon dioxide film 73 is deposited to a
thickness of about 1 .mu.m, parts of which are then selectively removed so
that openings 78 leading to the 25 respective n.sup.+ diffused layers 72
are formed therein.
Next, as shown in FIG. 11(c), tungsten is deposited on the n.sup.+ diffused
layers 72 in the openings 78 by selective CVD, so as to form tungsten
electrodes 74 having a thickness of about 500 nm. After a silicon dioxide
film having a thickness of about 1/2 of the shortest side of the opening
78 is formed over the entire surface of the substrate, the silicon dioxide
film is etched back so as to form sidewalls 79 on the side faces of the
openings 78.
Next, as shown in FIG. 11(d), aluminum containing 0.5% to 20% of silicon is
deposited on the tungsten electrodes 74 in the openings 78, so as to form
aluminum electrodes 75 in the same method as described in the fourth
embodiment. Thereafter, an aluminum film containing about 0.5% of silicon
is deposited over the entire surface of the substrate and then patterned
so as to form a large number of linear aluminum interconnections 76 which
are connected to the respective aluminum electrodes 75 and extend in the
direction orthogonal to the n+ diffused layers 72.
FIG. 12 is a view schematically showing an electric circuit of the
semiconductor memory device according to the present embodiment, wherein
reference numerals 80 and 81 designate a word-line select/drive circuit
and a bit-line select/drive circuit, respectively. From the word-line
select/drive circuit 80 extend a large number of word lines WL1, WL2, WL3,
. . . , while a large number of bit lines BL1, BL2, BL3, . . . extend from
the bit-line select/drive circuit 81. At the individual intersections of
the word lines WL1, WL2, WL3 . . . and the bit lines BL1, BL2, BL3, . . .
are disposed a large number of memory cells M11 M12, . . . each having the
same structure as the above unit cell Mc to form a matrix.
In the present embodiment, since each of the unit cells (memory cells)
functioning as a memory consists of the tungsten electrode 74 and the
aluminum electrode 75, the memory cell can be mounted directly on the
semiconductor substrate. Moreover, the process of silicidizing the
surfaces of the n.sup.+ diffused layers 72 functioning as interconnections
can be performed by a typical salicide process for MOS transistors.
Consequently, transistors in a peripheral circuit as well as memory cells
utilizing the resistance varying characteristic can be mounted on a single
semiconductor substrate, so that it becomes possible to easily fabricate a
semiconductor memory device containing rewritable non-volatile memory
cells.
Instead of titanium silicide, tungsten silicide, cobalt silicide, nickel
silicide, or the like may be formed on the surfaces of the n.sup.+
diffused layers 72. Although it is not necessarily required to silicidize
the surfaces of the n.sup.+ diffused layers 72, the resistance of the
n.sup.+ diffused layer 72 functioning as the interconnection can be
reduced more remarkably by silicidizing the surface thereof than in the
case where the n.sup.+ diffused layers are solely formed.
(Other Embodiments)
In the first to fifth embodiments, it is not necessarily required to form
the tungsten electrodes. It is sufficient to form, for example, a tungsten
film or a titanium film to a small thickness on the first aluminum
interconnections, while forming only the aluminum electrodes in the entire
openings. In this case, the sidewalls may be formed directly on the first
aluminum interconnections in the second to sixth embodiments. In the third
embodiment, the openings may be shifted in position from the first
aluminum interconnections. In the sixth embodiment, the aluminum
electrodes may be formed directly on the silicide layers so that the
sidewalls are formed on the silicide layers.
It will be appreciated that the vertical positions of the aluminum
electrode and tungsten electrode can be interchanged in each of the
embodiments.
It will easily be understood from the description of the memory functions
in each of the above embodiments that a tungsten plug may be interposed
between the first aluminum interconnection and the second aluminum
interconnection such that that portion of the first or second aluminum
interconnection which is contiguous to the tungsten plug functions as a
memory. In this case, that portion of the first or second aluminum
interconnection which is located in the vicinity of the tungsten plug will
form the resistance varying member according to the present invention.
An interconnection made of a material other than aluminum may be formed
instead of the aluminum interconnection used in each of the above
embodiments. It is not necessarily required to use aluminum as a material
composing the resistance varying member. Any material can be used provided
that it has such a resistance varying characteristic that its resistance
varies due to the movement of the constituent atoms caused by application
of a current.
Top